Dry termination for an electric cable

ABSTRACT

A termination for an electric cable includes a conductive element, a conductive shank, an insulating body, a casing of insulating material, and electric field control means. The conductive shank connects to an upper extremity of the conductive element and is adapted for connection of the termination to an electric installation. The insulating body includes a cylindrical portion surrounding a lower extremity of the conductive element, the cylindrical portion being adapted for connection of the termination to the electric cable, the conductive element and the insulating body constituting a rigid assembly adapted to resist a predetermined transverse stress. The termination also includes a joining assembly having an electric connection element connecting a conductor of the electric cable and the conductive element of the termination, and a flexible joint covering the electric connection element and a length of the cylindrical portion of the insulating body.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a national stage entry under 35 U.S.C. §371 fromInternational Application No. PCT/EP98/02063, filed Apr. 9, 1998, in theEuropean Patent Office; additionally, Applicant claims the right ofpriority under 35 U.S.C. §119(a)-(d) based on patent application No.M197A000817, filed Apr. 10, 1997, in the Italian Patent Office; thecontents of both of which are relied upon and incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a termination for an electric cable,for medium and high voltages, of the type for outdoor use, exposed toair and possible polluting agents, complete in all its parts beforebeing transported to installation place and suitable for connection tothe electric cable without accessing into the inside thereof.

For the purposes of the present invention, with medium and high voltagesit is generally meant voltages in a range of from 10 kV to 245 kV andmore.

For the purposes of the present invention, the term “termination for anelectric cable” means a device suitable to connect an electric cable toa bare conductor, such as, for example, an overhead conductor. Thistermination comprises electrical connecting means between the cableconductor and the bare conductor, and electrical separation meansbetween the outer surface of the cable and parts connected thereto,generally at ground potential, and the elements under tension, such asthe bare conductor. A termination of this kind generally comprises aconductive element, an insulating coating, and field control means ableto produce an electric field configuration compatible with dielectricstrength features of the relevant elements.

2. Description of the Related Art

A known termination for outdoor use is described in the publication “NewPrefabricated Accessories for 64-154 kV Crosslinked Polyethylene Cables”(Underground Transmission and Distribution Conference, 1974, pages224-232).

The termination comprises a supporting base for an outer insulatingcasing made of porcelain and provided with a finned surface. Theelectric cable devoid of its shield extends over the whole length of thetermination up to the upper extremity where it is fastened to besupported by the casing and where connection occurs to a voltage shankand therefrom to another electric installation.

A cylinder made of an epoxy resin incorporating a ground electrode isassembled about the elastomer element and is fastened to the supportingbase. The cylinder has a conical shape matching that of the cone-shapedelement inside it.

The installation further comprises the use of a spring system acting onthe lower end of the elastomer element so that a pressure between suchelement and the cable insulation takes place and also between theelement and the epoxy resin cylinder, so as to avoid the presence of airtraces and thus the risk of electrical discharges.

The space inside the porcelain casing is filled with insulating oil.

The termination, due to the presence of a porcelain casing, is of theself-bearing type and rests on a base being part of a pylon or the like.

Also known are terminations provided with a solid insulator of theoverhead type instead of resting on a supporting structure. According tothis solution the termination is associated with the structure byinterposition of an insulating element.

For example, from publication “IEE Power Cables and Accessories 10kV-180 kV”, London, November 1986, pages 238-241, it is known a 66 kVvoltage termination for an electric cable which is extended over thewhole length of the termination.

In this solution, starting from one cable end the semiconductive shieldis removed over a certain length thereof and the shield end is coatedwith a conductive paint until an insulator portion is covered.

On the area where the shield is cut off, heat-shrinkable tubes of givenimpedance due to the presence of capacitive and resistive elements areapplied, which tubes capacitively associated with the cable conductorreduce electrical stresses in the area where the shield is cut off.

An additional shrinkable tube is then disposed about the innermost tubesof same type and about the cable insulator portion.

The outermost tube comprises an insulating profile close to thecutting-off end of the cable shield.

A plurality of fins of heat-shrinkable insulating material is thenfitted on the outer tube.

For installation of the termination qualified workers skilled inheat-shrinking technique are required and accuracy, attention to detailsand cleanliness in all steps are needed.

Patent WO 91/16564, in its part describing the state of the art,mentions terminations made, as the previous one, according toheat-shrinkage technique, and explains how to avoid that technique bymaking use of a suitable mandrel on which a jacket of elastomer materialprovided with finnings is expanded; the jacket is contracted by slippingoff the mandrel onto a previously prepared electric cable of smallerdiameter.

The final cable portion to be enclosed within the termination comprisesthe cable conductor, followed at a certain distance by the insulatordevoid of a semiconductive layer and the shield.

A tubular winding is part of the cable assembling, made of a materialformulated so as to reduce stresses otherwise present at the end of thesemiconductive layer; this winding is applied on the semiconductivelayer and the insulator.

Alternatively, provision may be made for a layer intended for reducingsaid stresses within the inner insulating jacket, by coextrusion orrolling for example.

The upper end of the conductor is associated with a flat clamp providedwith a hole.

To make slipping the mandrel off easier, a plurality of ribs may beprovided on which the elastomer jacket rests, and also the use of alubricant between the ribs.

The mandrel is disposed with its elastomer jacket about the cable endportion already arranged to be part of the termination. The jackettwisting on the mandrel facilitates the lubricant spreading on the ribsand the mandrel slipping off, thereby causing contraction of the jacketon the cable end portion.

Also known are terminations for electric cables in which the electriccable is inserted only partially into the termination instead ofextending up to the shank connecting to the overhead line or otherelectric installation.

One example of such self-bearing termination is described in patentapplication EP 95 101 338.2 of the Applicant itself.

In this solution, the electric cable is stopped at a given point fromthe termination entrance and the electric connection over the wholelength which is required to reach the overhead line is accomplished bymeans of a conductive element.

The conductive element is externally provided with a finned elastomerlayer and is supported by an insulating base preferably made of an epoxyresin resting on a supporting element.

The assembly consisting of the epoxy resin base and the conductiveelement makes the termination of the self-bearing type.

In particular, the assembly consisting of the epoxy resin base and therigid conductive element, brings the electric field to the terminationsurface and to a value compatible with the dielectric strength of airthat is long enough to form a leak line adapted to resist discharges.

The electric cable, at a given distance from the termination entry, isdeprived of the shield, leaving the insulator uncovered. At the shieldcutoff, electric field control means are provided which consist of adeflecting cone and an upper insulator useful for giving rise toelectric field values compatible with the electrical strength of theemployed materials and the surrounding air.

The epoxy resin base comprises a cone-shaped cavity on which the endportion of the insulator of the field control means rests.

A suitable spring arrangement pushes the cone and the insulator over itagainst the cone-shaped surface of the resin base in such a manner thatno incorporation of air bubbles may occur between the contact surfacesof the different parts, which as known may bring about risks ofelectrical discharges.

WO 97/09762 discloses a cable termination for a high-voltage cableinsulated by a solid insulating material which is used to connect thecable with an insulated conductor. With a particular arrangement ofelectrical couplings and field-controlling elements between the cableconductor and the insulated conductor, this device allows a rapid andleakage-free installation forming a closed current path without the riskof flashover. That device is primarily (even though not exclusively)designed for connecting solid-insulated cables with gas-insulateddevices. In particular, in FIG. 2 a connection between a solid-insulatedcable and a gas-insulated insulator for outdoor use is illustrated.

BRIEF SUMMARY OF THE INVENTION

The Applicant has noted that, by arranging a cable termination completedin all its parts before being mounted into its support and location ofuse, and by subsequently carrying out the operations of connection tothe cable, installation operations can be substantially simplified andreliability of the operating assembly increased. Such connection to thecable operations employ connection, junction, or lapping techniquesfalling within the standard technical knowledge of the installationuser, generally an electric energy supplying company or the like.

Within the scope of the present invention it has been found, inparticular, that a termination having a portion substantially of thesame size as the cable for which the termination is intended allowsready connection, by means of known techniques, of the termination tothe cable. In this way, all assembly operations and technical tests onthe termination could be carried out at the factory, in a controlledenvironment and by a qualified staff, without requiring performance ofthese operations in the field or by the customer.

In a first aspect, the invention relates to a termination assembly foran electric cable, comprising:

a termination having a conductive element and an insulating coveringextended over it and

means for connection to an electric cable, characterized in that

said termination comprises a connecting portion comprising a conductorand an insulating covering coaxial thereto, having a substantiallycylindrical configuration and predetermined lengths and diameters, and

said connecting means comprise a joining assembly which connects saidconnecting portion of the termination with one end of said electriccable,

wherein said predetermined lengths and diameters are compatible withsaid joining assembly.

In a preferred embodiment, said insulating covering of the terminationcomprises an insulating body having means for connection with an outersupporting structure.

In particular, said connecting portion of said termination comprises anend portion of said conductive element and said insulating body.

Preferably, said joining assembly connecting said connecting portion ofthe termination and one end of said electric cable comprises an electricconnection element, joining a conductor of said electric cable and saidconductive element of said termination, and a prefabricated flexiblejoint covering said electric connection element and a portion of saidconnecting portion of the termination and of said end of said electriccable.

In another aspect, the present invention relates to a termination for anelectric cable of predetermined diameter, comprising:

a conductive element longitudinally extended between one lower extremityand one upper extremity,

a conductive shank connected to said upper extremity of said conductiveelement, adapted to connect the termination to an electric installation,

an insulating body surrounding said conductive element,

a casing of insulating material about said insulating body,

field control means included in said casing, characterized in that saidconductive element comprises a substantially cylindrical portionincluding said lower extremity, and said insulating body comprises aportion surrounding said lower extremity of the conductive element,having an outer cylindrical surface coaxial with said substantiallycylindrical portion of said conductive element.

In particular, said insulating body comprises a conductive coatingextended over a predetermined length of said portion surrounding thelower extremity of the conductive element, suitable to constitute anelectric shield about the conductive element itself.

Preferably, said field control means are in contact with said conductivecoating of the insulating body.

In a preferred embodiment said field control means comprise a fielddeflecting element of semiconductive elastomeric material.

Preferably, said conductive coating on the insulating body comprises apaint layer filled with conductive fillers.

Preferably, said insulating body comprises means for connection to anouter supporting structure.

In particular, said connecting means comprise a flange transverse to thelongitudinal axis of the termination comprising a supporting surface forthe terminal to the supporting structure.

Preferably, in an aspect of the present invention, said conductiveelement and said insulating body constitute a substantially rigidassembly, adapted to resist a predetermined transverse stress.

Preferably, said casing of insulating material about said insulatingbody comprises a portion of elastomeric material adhering to theinsulating body or elastically tightened on the insulating body.

Preferably said casing of insulating material comprises an outer partprovided with a finned surface; preferably said outer part with a finnedsurface consists of a material having environmental resistance.

In a preferred embodiment, said first part of the insulating assemblyincorporates a deflecting element elastically pushed along the lowerportion of the insulating body in contact with the conductive coatingapplied to the insulating body forming an electric shield.

In a particular embodiment, said insulating body incorporates adeflecting element in the lower part thereof and said insulating casingis formed of a finned tube elastically and directly fitted on the outersurface of the insulating body.

In a further aspect, the present invention relates to a method ofconnecting an electric cable, comprising an electric conductor, aninsulating covering and a shield disposed about said conductor, to abare conductor, characterized in that it comprises the steps of:

suitably arranging a termination comprising a conductive element, aninsulating body about said conductive element, a casing of insulatingmaterial on said insulating body and a conductive shank connected to afirst extremity of said conductive element;

arranging a conductive coating adapted to form an electric shield over apredetermined extent of the lower portion of the insulating body,

providing electric field control means at an extremity of saidconductive coating of said insulating body, and subsequently

carrying out a mechanical and electric connection between a secondextremity of the conductive element of the termination and the cableconductor,

insulating said mechanical and electric connection,

connecting the cable shield to said conductive coating of thetermination.

Preferably, the method of the invention further comprises the step ofmechanically connecting said insulating body to a supporting structure.

Preferably, said steps of insulating said mechanical and electricconnection and connecting the cable shield with said conductive coatingof the termination comprise applying a joint over the area correspondingto said mechanical and electric connection between a second extremity ofthe conductive element of the termination and the cable conductor.

In a particular embodiment, said step of arranging a conductive coatingadapted to form an electric shield over a predetermined extent of thelower portion of the insulating body comprises applying a semiconductivepaint to the insulating body.

Preferably, said electric field control means have at least one portionpressed on the conductive coating of the insulating body.

In a preferred embodiment said casing is elastically fitted about saidinsulating body.

In particular, in accordance with the method of the invention, saidsteps of:

suitably arranging a termination comprising a conductive element, aninsulating body about said conductive element, a casing of insulatingmaterial on said insulating body and a conductive shank connected to afirst extremity of said conductive element;

arranging a conductive coating adapted to form an electric shield over apredetermined extent of the lower portion of the insulating body,

disposing electric field control means at an extremity of saidconductive coating of said insulating body, are carried out at thefactory before installation of the termination in the field.

Preferably, the method of the invention further comprises the step ofelectrically testing said termination carrying said conductive coveringand electric field control means at the factory, before installation.

In accordance with a further aspect, the present invention relates to amethod of manufacturing a termination for an electric cablecharacterized by the steps of:

arranging a conductive element linearly extended over a predeterminedlength,

applying an insulating body about the conductive element,

externally applying a conductive coating over a predetermined extent toa portion of the insulating body close to one of the extremities of theconductive element,

applying electric field control means in electrical contact with saidconductive coating,

disposing a casing of insulating material having a predetermined surfaceextension over said insulating body and over said electric field controlmeans, in an area corresponding to the extremity of said conductiveelement and said electric field control means.

Further features will become more apparent from the detailed descriptionof some preferred embodiments of a termination for an electric cable inaccordance with the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

This description will be given hereinafter with reference to theaccompanying drawings, in which:

FIG. 1 is a longitudinal sectional view of a termination in accordancewith the invention, connected to an electric cable,

FIG. 2 is a longitudinal sectional view of the termination alone shownin FIG. 1;

FIG. 3 shows a detail of FIG. 2 in an expanded scale;

FIG. 4 is a longitudinal sectional view of an alternative embodiment ofa detail of the termination of FIG. 1;

FIG. 5 shows the joint of FIG. 1 in an expanded scale;

FIG. 6 shows the field line configuration in a portion of thetermination of FIG. 1 in use conditions,

FIG. 7 is a longitudinal sectional view of a first embodiment of anapparatus for verifying the operability of the termination of FIG. 1;

FIG. 8 is a longitudinal sectional view of another embodiment of anapparatus for checking the operability of the termination of FIG. 1.

DETAILED DESCRIPTION OF INVENTION

An example to carry out the invention is shown FIG. 1, comprising atermination 1 associated with a supporting structure 2, an electriccable 3 and connecting means or joint 4 between cable 3 and termination1.

The upper end of the termination is connected to an overhead line 3′,diagrammatically shown in FIG. 1 by a dashed line.

The electric cable 3, generally for voltages included between 60 and 500kW (this voltage range being generally designated as “high voltage” inthe art) comprises a conductor 5, for instance a copper cord, anextruded insulator 6 and a semiconductive layer 7, stepwise removed inits end portion for connection with joint 4.

Layer 7 forms the shield inside which the electric field lines generatedby the conductor 5 voltage are guided.

The electric cable can be of a different type with respect to theexemplified one; for instance, it may have a layered structure, made ofpaper or paper laminate and polymeric material soaked with oil orinsulating mixtures.

Termination 1, better shown in FIG. 2, where it is illustratedseparately, complete in all its parts before being associated withsupporting structure 2 and electric cable 3, comprises a conductiveelement 8 extended between the upper and lower ends 9, 10 of thetermination; an insulating body 11 adhering to the conductive element; aconductive coating 12 applied over a predetermined extent of the outerlower portion of the insulating body 11; a field control element 13; anda finned casing 14 of insulating material extended from the upper end 9of the termination, in proximity to the termination portion to beassociated with the supporting structure. In the example of FIG. 2, inparticular, the finned casing 14 extends up to covering field controlelement 13 which, in turn, is in contact with the upper edge 15 of theconductive coating 12.

In the following, the expression “conductive element” means either anelement completely made of a conductive material, of solid or tubularstructure, flexurally rigid, or a flexible conductive element madesufficiently rigid for the purposes of the invention by further elementsassociated therewith, as shown in the following, or any other equivalentform, such as an element of dielectric material with which a tubularconductive or conductive-made layer is chemically or mechanicallyassociated.

In some preferred embodiments the conductive element is of metalmaterial, such as steel, copper, aluminium or any other metal of goodelectric conductivity.

By field control element it is herein intended either a suitably tracedconductive body, usually designated as electrode or deflector, currentlymade of a semiconductive elastomeric material, incorporated into a bodyof insulating material, the whole being designed so as to keep theelectric field gradient within acceptable limits, or a material havingpermittivity and conductivity variable according to the electricgradient, or also a condenser system, designed for the same purpose.

The termination comprises a shank 16 at the upper end which is connectedto the conductive element 8 and projects outwardly for connection withthe electric overhead line 3′; the shank 16 is surrounded by a shieldring 17 of a size such as to avoid discharges due to “corona effect”.

According to a preferred embodiment, the insulating body 11 about theconductive element can be formed of a unitary piece, as shown in theleft-hand portion of FIG. 2, for instance made of epoxy resin,optionally comprising fibres or other fillers to increase mechanicalstrength.

Alternatively, according to an embodiment shown in the right-handportion of FIG. 2, the insulating body 11 is made, for a first portion11 a, of an epoxy resin, and for the remaining portion 11 b of a layer18 of insulating elastomeric material, for instance EPR, silicone rubberor the like.

Preferably, the unit formed by conductive element 8 and insulating body11 comprises the supporting elements intended for combining termination1 with supporting structure 2 shown in FIG. 1.

As more clearly shown in the expanded view of FIG. 3, said supportingelements are preferably made by casting the insulating body in a singlepiece comprising a flange 19 placed at a distance of predetermined value“a” from the lower extremity 10 of the termination; for example, for atermination at 170 kV a value “a” of about 500 mm may be appropriate.

In the right-hand portion of FIG. 3, fastening elements 20 forinsulating body 11 are partially shown, with dashed lines, which in thisexample consist of a pair of opposite plates 21 a, 21 b, pressed at theopposite sides of flange 19 by anchoring tie-rods 22.

Said fastening elements 20 further comprise bolts 23 and nuts 24 tofasten plate 21 b to a base 25 of the supporting structure 2, as shownin FIG. 1.

The conductive coating 12 applied to the lower portion of the insulatingbody is intended to constitute a shield inside which the electric fieldlines are contained.

Preferably, the conductive coating is obtained by a semiconductive paintapplied both to the tubular portions of insulating body 11 downstreamand upstream of flange 19 and to the upper surface of the flange itself.In one embodiment, the paint comprises an insulating resin filled withconductive components, for instance, an epoxy resin filled withgraphite. Alternatively, a semiconductive taping wound tightly about theinsulating body, a heat-shrinkable elastic tube filled with carbonblack, an aluminum tube with a horizontal flange, or another equivalentsolution adapted to form an electric shield about the conductive element8 may be provided.

Field control element 13 when the termination is in use is intended forconfining the distance of the electric field lines emerging from upperedge 15 of conductive coating 12 to values compatible with the strengthof the surrounding air.

According to the preferred embodiment of FIG. 2, the field controlelement 13 comprises a deflecting body 26, of semiconductive elastomericmaterial, incorporated into the lower portion of an insulating sleeve 27covering insulating body 11.

Insulating sleeve 27 is constituted by an insulating elastomeric blend,for instance EPR (elastomer based on an ethylene-propylene copolymer, oran ethylene-propylenediene terpolymer), devoid of conductive fillers,and deflecting body 26 is constituted by a semiconductive blend, forexample an EPR-based blend made conductive by carbon black fillers.

By insulating blend it is herein meant an elastomeric blend having aresistivity substantially not lower than 10¹² Ωcm; by semiconductiveblend it is herein meant an elastomeric blend having a resistivitysubstantially not exceeding 10⁴ Ωcm.

Finned casing 14 is formed of an insulating material, elastically fittedon sleeve 27. Alternatively, sleeve 27 itself may be provided with anouter finned surface, if the constituting material is suitable to theintended use conditions. In this case finned casing 14 forms one portionof sleeve 27.

Finned insulating casing 14 is suitable to resist the surroundingatmosphere, therefore it does not show, during use, the so-called“tracking” phenomenon described in standard IBC 1109, 1992 as anirreversible degradation of the insulating material surface, involvingformation of conductive paths even in dry conditions.

According to the embodiment of FIG. 2 the finned insulating casing 14extends until it covers the lower portion of deflecting cone 26; inother embodiments of the invention the casing could extend in adifferent manner from that shown in the figure, depending on specificuse requirements.

FIG. 4 shows a second embodiment of the termination; in this figureparts similar to those of the termination of FIG. 2 are indicated by thesame reference numbers.

In the embodiment of termination shown in FIG. 4, insulating body 11′incorporates field deflecting element 26.

Insulating body 11′, analogously to what already described forinsulating body 11 in the embodiment of FIG. 1, can be formed of aunitary piece, preferably made either of an epoxy resin reinforced withfibres or similar fillers, or of another insulating material, as shownin the left-hand side of FIG. 4; alternatively, as shown in theright-hand side of FIG. 4, it may consist of a lower part 11′a made ofan insulating material, for example an epoxy resin, and of an upper part11′b made of a layer of elastomeric insulating material, for instanceEPR.

In both the above solutions a finned insulating casing 14 is present,which is of substantially tubular form, having a low thickness, made ofan elastomeric material associated with the inner cylindrical resinsurface of the insulating body. Conveniently, this casing, besidesgiving a sufficiently extended finned surface so as to avoid surfacedischarge phenomena, offers a protection to the insulating body againstweathering agents, such as humidity and the like.

By way of example, with reference to measurements shown in FIG. 2, andcorrespondingly as regards the embodiment of FIG. 4, for a terminationup to 170 kv there is an overall height H between the shank and thelower extremity of about 2500 mm, indicating with H the sum between thealready mentioned distance “a” equal to about 500 mm and the distance“h” between shank 16 and the upper surface of flange 19, equal to about2000 mm.

Value “h” is determined by the requirement of sufficiently spacing apartfrom each other shank 16 under tension from flange 19 associated withthe supporting structure, electrically connected to the earth, in such amanner that the finned surface of casing 14 may give rise to a leak linesuitable to avoid electric discharges to earth.

In the described example, the value of the leak line is about 4000 mm.

Value “a” is determined by the requirement of having a sufficientavailable space under the flange 19 in order to be able to validly carryout an electric or mechanical connection between the termination and theelectric cable.

For this purpose, the unit formed by the conductive element and theinsulating body is already suitably arranged for a subsequent connectionwith the electric cable.

In particular, as shown in FIG. 3, conductive element 8 has, inproximity to its lower extremity, a portion 10 a not covered byinsulating body 11; in turn, insulating body 11 is provided with anannular groove 28, the whole being suitable to allow an easy connectionwith the cable, as explained in the following.

In the embodiment shown in FIGS. 1 to 3, advantageously the diameter ofthe cylindrical portion of the insulating body in correspondence withits portion disposed above flange 10, has a value greater than that ofthe corresponding extension under the flange itself.

Additionally, as shown in FIG. 3, also the diameter of conductiveelement 8 in correspondence with its part 8 a disposed above flange 19,is a value greater than that of the corresponding extension 8 b underthe flange itself.

In this way, the self-bearing capability of the conductive element isincreased, which will result in a greater resistance to forces actingtransversely to the termination, and the electric field lines distributeover a larger section of the insulating body.

In the example of FIG. 2, for voltages up to 170 kV, the conductiveelement is made of aluminum (hardened avional) and has diameter “d₀” of50 mm. The diameter of the insulating body has a value “d” equal to 200mm. The values of diameter “c” of the lower portion of element 8 and ofthe corresponding diameter of the extension of the insulating body underflange 19 are determined by the cable size for which the termination isintended and are advantageously equal or similar to the correspondingdiameters of the conductor and of the cable insulator to be connected.

Termination 1 of FIGS. 2 and 4 constitutes a complete unit in all itsoperating parts, irrespective of the electric cable for which it isintended.

Therefore, termination 1 may be manufactured and tested in anindependent manner and subsequently transferred to the installationplace to be connected to the supporting structure and to the relevantcable.

FIG. 1 shows termination 1 in its installation position on supportingstructure 2, its flange 19 being sandwiched between opposite plates 21a, 21 b mutually tightened by tie-rods 22, and the upper plate 21 bfastened to the base 25 of the structure by a bolt and nut arrangement23, 24.

The termination is connected to the upper part thereof to the overheadline 3′ by shank 16.

The method of connecting the termination to the cable in the mostgeneral aspect of the present invention comprises the followingfundamental steps:

a) disposing cable 3 externally and at a lower position with respect totermination 1 after removing a portion of insulator 6 from the centralconductor 5 end and a portion of shield 7 from said insulator 6;

b) mechanically and electrically connecting the upper extremity of cableconductor 5 to the lower extremity 10 of conductive element 8;

c) connecting cable shield 7 to conductive coating 12 of insulating body11.

Advantageously, steps a, b, c can be carried out by usual operatorsskilled in the field of electric cables, thereby avoiding the usualknown operations, wherein introduction of the cable into a terminationis required partially or over the whole length thereof, withintervention of skilled persons acquainted with the specific features ofthe termination.

Connection of the cable to the termination, in particular restoring ofinsulation and shield at the junction area may comprise techniquesusually adopted to connect two cable lengths, for instance either byapplying tapes or the like or, preferably, by using prefabricatedelastic joints, as described in the following as an example and shown inFIG. 1 and in FIG. 5 to an expanded scale.

Joint 4 comprises a terminal 29 for connecting cable conductor 5 andconductive element 8 of the termination together, a metal clip 30,substantially tubular and made of two detachable parts, whoseextremities 31, 32 are inserted into groove 28 of insulating body 11 ofthe termination and into a suitable groove 33 formed in cable insulator6 respectively; clip 30 aims at avoiding relative movements by effect ofheat-shrinkage of the respective materials, as described in more detailin Patent EP 0 199 742.

Terminal 29 and clip 30 are suitably electrically connected by aconductor wire 34, so as to determine an equipotential condition andconsequently an absence of electric discharges even in the presence ofair.

The joint further comprises a field-deflecting body 35, made ofelectrically conductive material, a cover 36 made of premouldedinsulating material, and a sheath 37 made of semiconductive material,whose ends are respectively in contact with cable shield 7 andsemiconductive coating 12 of insulating body 11.

A copper braiding 37′, which is also in electric contact with cableshield 7 and semiconductive coating 12, surrounds the joint to avoidinfluence of capacitive currents; furthermore, one or more outersheaths, not shown, cover the assembly.

FIG. 1 further shows earthing elements of the supporting structure 2 ofthe termination.

These elements comprise a copper plait 38, applied between plate 21 aelectrically connected to base 25, and a collar 39 tightened on thecable shield 7; a suitable electric cable 40 brings collar 39 to groundpotential.

A sheath 41, made of polyethylene or the like, is disposed externally ofcable shield 3, having the function of protecting the cable metal shieldagainst corrosion that could take place in case of water penetration.

In the embodiment of FIG. 1, the sheath integrity can be checked, forexample as regards the possible presence of holes in the sheath itself.

This check is made by applying electric voltage to the cable metalshield, for example by a hand-held generator, and verifying itsinsulation relative to the earth. To carry out this test, connections 38and 40 are temporarily removed, so that shield potential can be raisedrelatively to the ground potential for a period necessary for the check.

To this purpose, an interruption in the continuity of the semiconductivepaint layer 12 is present in correspondence to flange 19, between thelower portion of body 11, on which joint 4 is fitted, and the upperportion of body 11, so as to electrically separate the cable shield thatis to be energized from support 2, which is at ground potential.

Termination 1 connected to joint 4 operates as described hereinbelow.

The termination is characterized in that it comprises a cable mock-upinside it, which is formed by the conductive element associated with theinsulating body to the lower portion of which, both upstream anddownstream of flange 19, the semiconductive paint 12 suitable to form anelectric shield is applied.

The electric field lines are included between the maximum potentialvalue of the conductive element and the potential value of the earthedshield (consisting of semiconductive paint 12).

In correspondence with the shield interruption, i.e. in correspondencewith upper edge 15 of semiconductive paint layer 12, a transition froman area in which the electric field lines are within insulator 11 and anopen area occurs; in this area a density increase in equipotentialelectric field lines would occur in radial direction, because said uppershield edge, being corner-shaped, has high values of curvature.

As known, in a termination the value of the electric field gradient in agiven area is substantially expressed by the ratio between electricpotential at said area and distance between adjacent equipotentiallines.

Therefore, in correspondence with the upper edge of the shield anexcessive value of the field gradient would be present which will bringabout the risk of electric discharges.

In accordance with the embodiment of FIG. 1, said density increase ofthe electric-field lines is avoided due to the presence of deflectingcone 26 incorporated into insulating sleeve 27.

In the embodiment of FIG. 2, the sleeve 27 is advantageouslycharacterized by a section widening at the lower portion relative to theremaining portion, both for the purpose of enabling incorporation of adeflecting cone 26 of appropriate size and in order to determine asufficient size so as to cause a mutual radial moving apart between theelectric field lines, in order to obtain an electric gradient value atthe outer surface which is compatible with the rigidity of the employedmaterials and the surrounding air.

In the preferential embodiment shown, for voltages up to 170 kV, thediameter corresponding to section widening of sleeve 27 has a value “D”corresponding to 440 mm where the diameter values of the conductiveelement 8 and insulating body 11 are those previously mentioned.

By way of example, on the left-hand side of FIG. 6, the course of theelectric-field lines is qualitatively shown, as well as the relevantspacing determined by field-control means, adapted to operate atelectric strength values in the air lower than 2.5 kV/mm and preferablylower than 2 kV/mm.

A good operation of the termination further depends on the heightbetween shank under tension 16 and flange 19 in contact with thesupporting structure at the ground potential.

To this end, the self-bearing features of the termination, determined byits rigid central portion supported through flange 19 on the supportingstructure, enable accomplishment of a termination, suitable to operatein an upright position, having a length such as to provide a path in theair long enough so as leak current has a negligible value.

Because of its rigidity to bending the termination can operate withoutremarkable deformations and in the presence of transversal stresses.

The presence of a great number of finnings on the insulating sleeve 27helps in supplying a leak line of a sufficiently high value to avoidelectric surface discharges.

The conditions for determining this leak line are specified, forinstance, in publication 815 of 1986 by International ElectrotechnicalCommission having title “Guide for the selection of insulators inrespect of polluted conditions”.

The height value “h1” of the termination together with the shape andsize of the fins on casing 11 leads to the above mentioned value of theleak line.

As shown in FIG. 1, at the passage between upper and lower surfaces offlange 19 an interruption of shield 12 can be seen.

However, accomplishment of termination 1 contemplates the possibility ofgiving flange 19 such thickness values that the path of the electricfield lines does not substantially diverge from the path that wouldoccur in the presence of a shield at this area too, except for a slightradial swelling, qualitatively shown in FIG. 6, to which, asascertained, no risk of an excessive value of electric gradientcorresponds.

It is pointed out that in a termination up to 170 kV, having sizes asabove described in the example, an appropriate thickness for flange 19is of about 40 mm.

The course of the electric-field lines at joint 4, in the presence ofdeflecting body 35 and the semiconductive material sheath isqualitatively shown in FIG. 5.

A correct operation of the termination depends on a variety of features,determined in accordance with the invention in the terminationmanufacturing process even before its mounting on supporting structure2.

A first essential condition for correct operation of the termination isthe absence of air between the different parts subjected to highelectric potential differential, in particular the absence of airbubbles between conductive element 8 and insulating body 11 over theirwhole extension, between conductive coating 12 and insulating body 11,between deflecting cone 26 and insulating body 11, between finned casing14 and insulating sleeve 27.

Actually, the presence of air, exposed to ionization phenomena in thepresence of an electric field, would trigger electric discharges fromthe portion at greater potential towards that at ground potential.

A convenient result, as regards operation, has been achieved in thefollowing manner.

The insulating body 11 is made perfectly adhering to the conductiveelement 8, for example by a moulding process comprising the steps ofdisposing the conductive element in a mould and pouring or extrudingaround it the material intended to form the insulating body itself, ormaking a winding with insulating fibres or yarns (such as fibreglass orpolyaramidic fibres and the like), disposed about the conductive elementand soaked with a resin, for example a thermosetting resin (polyester,epoxy resin or the like).

Adhesion of semiconductive shield 12 to a predetermined portion of theinsulating body is obtained, for example, by making the shield itselfthrough application of a semiconductive paint on the relevant portion ofthe outer surface of body 11.

Deflecting body 26 adheres to the conductive coating 12 because it isconveniently made of an elastomeric material and is incorporated intosleeve 27, the whole being elastically fitted on insulating body 11, insuch a way that the deflecting body exerts pressure on thesemiconductive coating, for example by an interference degree of atleast 10% between sleeve 27 and insulating body 11.

Alternatively, sleeve 27 may be directly moulded on body 11, afterarranging deflecting body 26 thereon; in this case, adhesion betweenelements can be promoted by interposition of an adhesive material.

Finned casing 14 is made adherent to the surface of insulating sleeve27.

In an embodiment, finned casing 14 is applied to insulating sleeve 27after being moulded separately, by elastic fitting of the former on thelatter.

Alternatively, when the employed materials have suitable features,finned casing 14 may be an integral part of the sleeve itself, formingthe outer portion of same; the finned profile may be made either bymoulding or by mechanical working, i.e. by chip removal, grinding,refacing and the like.

Alternatively, finned casing 14 can be moulded around insulating body 11by adopting a finned mould, suitable to be filled, for instance, withliquid silicone; this technique allows to obtain both only the partforming the finned casing, or, optionally, the whole assembly consistingof insulating sleeve 27 and finned casing 14, which, in the lastmentioned case, will form the outer portion of said sleeve.

A termination in accordance with the present invention can beadvantageously submitted to a final quality control method so as tooffer the customer the assurance of a perfect operation in use, withoutany additional check.

By way of example, it is hereinafter described a first quality controlmethod for the termination, with reference to the embodiment shown inFIG. 2. To this purpose, the termination, constructed as previouslydescribed, is completed upon mounting it on the upper extremity ofshield ring 17 and shank 16.

As shown by test diagram of FIG. 7, the method uses a bulb 43 with aconductive surface, and a metal container 44 filled with an insulatingfluid 44 a.

The method comprises the steps of:

disposing the bulb 43 about the end of the conductive element 8;

introducing the end portion comprising bulb 43 into the insulating fluidwithin metal container 44 electrically connected to the earth and at apredetermined distance from the container walls, for test uniformitypurposes;

applying a voltage of a predetermined value between shank 16 of thetermination and earthing 45 of the container.

In the example, the bulb has an outer diameter of 300 mm, the bulbsurfaces are at a distance of 350 mm from the side walls of container44, the insulating fluid is a mineral oil; the test voltage correspondsto the use voltage of the termination and can be increased up to 250% ofthe use voltage itself.

The positive result of the test corresponds to the absence of electricdischarges up to said maximum test voltage.

The above described method allows to provide the customer with analready guaranteed termination for a subsequent good operation, and alsocompleted in all its parts and arranged for subsequent connection to theelectric cable.

According to an alternative method, the quality control of thetermination in accordance with the invention is accomplished by a testas diagrammatically shown in FIG. 8.

To this purpose, a conductive bar 46 is arranged, which advantageouslyhas the same diameter of conductive element 8 and of the cable conductorfor which the termination is intended, provided at the lower extremitythereof with a ball 47, the whole being incorporated in an insulatingresin mass 48, having an upper portion 48 a of same size as the cableinsulator for which the termination is intended.

The resin mass is coated with a semiconductive layer 49 at apredetermined distance from ball 47, for example a distance of about 100mm. Therefore, the method comprises the steps of:

connecting the semiconductive layer 49 to an earthing installation 50,

connecting, by a terminal 29 suitable to give the electric contact, theupper extremity of conductive bar 46, projecting from the insulatingupper portion 48, to conductive element 8 of the termination,

applying a joint 4 over terminal 29, thereby connecting semiconductivelayer 49 of resin mass 48 to conductive coating 12 on insulating body 11of the termination;

applying a voltage between termination shank 16 and earthinginstallation 50.

Preferably, joint 4 of FIG. 8 is of the already described type, withreference to FIG. 5.

In accordance with this solution, description of the joint is omittedfor the sake of simplicity and the different joint parts are denoted bythe same reference numerals as in the previous type.

Acceptability of the termination is determined by the absence ofelectric discharges until the maximum voltage values and according tothe modalities required by test standards.

The method allows to check good functioning both of the termination andof its connection to the joint, with advantages for the customer inparticular when the joint employed in the test is prefabricated and ofthe same kind as provided for use in the final installation, as shown inFIG. 1.

The termination, whose functioning is guaranteed by the above describedtest methods, does not require during installation any further operationinside it, so that it keeps configuration of the different componentsand achieved features unchanged in time.

As compared with the termination described in the already mentionedpatent application EP 95 101 338.2 of the Applicant itself, thetermination in accordance with the invention is convenient.

Also for the termination of the above mentioned patent application aquality test may indeed be envisaged after manufacturing and beforeinstallation, by introducing a cable sample into it following knownmodalities, which sample will be identical with the one to be actuallyused for the final termination installation.

However, in this embodiment, specific mounting operations are needed tocarry out the required fastening conditions of the relevant fieldcontrol elements, which operations greatly affect the test result and inany event must be repeated anew when the termination has been installed.

Therefore, the test and subsequent installation steps for this knowntermination require repeated interventions inside the terminationitself, with very delicate operations and calibrations, as well assophisticated assembling techniques typical of operators skilled in thefield of terminations.

Conversely, the termination in accordance with the present inventionuses an assembly extended lengthwise over the whole termination length,formed of the conductive element 8 and body 11, on which firstlyinsulating sleeve 27, wherein deflecting body 26 is incorporated, andthen finned casing 14 are fitted; in this way a perfect adhesion betweenthe various elements is achieved which is necessary to prevent airbubble incorporation. All these operations are carried out at thefactory, under controlled conditions and by qualified staff, and issubsequently followed by specific test operations.

The present invention is therefore distinguished from the preceding oneand also from terminations made of porcelain and provided withinsulating oil as described in the prior art, not only for the abovedescribed solutions according to the invention, but also because it isnot necessary to use traditional spring pressing systems which implysubsequent adjustment operation.

The termination according to the invention keeps during time adhesionobtained at the factory and hence allows connection of the terminationitself to the electric cable without requiring further specific tests.

Furthermore, the termination in accordance with the invention isself-bearing in an upright position, even in the presence of transversalstresses, thereby avoiding the use of conventional self-bearingstructures having an outer porcelain casing and an insulating oil in itsinterior, which results in advantages for the absence of explosion orfire risks in the presence of inner electric discharges.

It is to be noted in particular that, although in the precedingdescription the conductive element 8 has been defined as “rigid”,rigidity of the whole is ensured by the assembly consisting of element 8and body 11; therefore, if body 11 is suitable to ensure by itself thedesired rigidity features to the termination, for example being made offibre glass reinforced resin or the like, the only function required toelement 8 will be that of electric conductor.

Alternatively, mechanical rigidity features can be shared, based onspecific use requirements, between element 8 and body 11, and optionallyother existing elements, until the desired rigidity of the assembly isachieved.

What is claimed is:
 1. A termination for an electric cable, comprising:a conductive element longitudinally extended from a lower extremity toan upper extremity; a conductive shank connected to the upper extremityof the conductive element, adapted for connection of the termination toan electric installation; an insulating body surrounding the conductiveelement; a casing of an elastic insulating material about the insulatingbody; and electrical field control means included in the casing; whereinthe insulating body comprises a substantially cylindrical portionsurrounding the lower extremity of the conductive element, thesubstantially cylindrical portion being adapted for connection of thetermination to the electric cable, the conductive element and theinsulating body constituting a substantially rigid assembly adapted toresist a predetermined transverse stress, further comprising a joiningassembly comprising an electric connection element connecting aconductor of the electric cable and the conductive element of thetermination, and a prefabricated flexible joint covering the electricconnection element and a length of the substantially cylindrical portionof the insulating body.